In the process of assembling a turbocharger rotor, a turbine wheel and a shaft are typically welded to each other, such as by electron beam and laser welding. In order to remove internal residual stresses generated by the welding, it is known to use a “welding stress relieve” process in which the weld is heated up to a high temperature, and then control-cooled to a lower temperature.
It is known to use induction heating to stress relieve the weld between the turbine wheel and its rotor shaft. This induction heating process entails heating the weld by electromagnetic induction, through heat generated in the area around the weld by eddy currents. The eddy currents are generated by an electromagnet that is driven by an electronic oscillator passing high-frequency alternating current through the electromagnet.
This induction heating process generates heat inside the weld area itself, rather than generating it externally and applying it to an outer surface. Thus, the weld area can be heated rapidly. This process provides numerous benefits, including good heat uniformity and quality, and reduced cycle time. More particularly, because induction heating heats from within, it increases uniformity, and reduces the time needed to uniformly reach the desired temperature. The uniformity leads to better quality. Induction heating is also safe, reliable, easy to use, and power-efficient. For example, stress relieving through an induction method is worker-friendly in that it does not require the handling of hot heating elements and connectors.
Nevertheless, the use of the induction stress relieve process is a source of hardness degradation on an already hardened rotor shaft. More particularly, the heating of the region of the weld area leads to conductive heating of the hardened rotor shaft. This can lead to a degradation of the hardness of the journal bearing area and A Datum surface (i.e., the axial bearing surface) of the shaft, which can increase the noise created by operation of the turbocharger due to a weakened bearing area of the rotor shaft.
Accordingly, there has existed a need for a way to stress relieve the connection between a turbocharger turbine wheel and a rotor shaft without degrading the hardness of the rotor shaft.